For the second time in three years, the board of Greenpeace U.S.A. is resigning, unable to resolve serious rifts over policy. The organization suffered a major shake-up in 1997, when, in the face of declining membership and revenue, it ended its grassroots tradition of going door to door for members and donations, shrank its national staff by 75 percent, ousted its executive director, and dissolved its board. Greenpeace U.S.A. has had something of a comeback in the past year, with successful campaigns against the sale of redwood furniture and a high-profile presence in Seattle this week protesting genetically modified foods. But the staff has seen high turnover in recent months, one staffer says, partly because of difficult relations with the current executive director, Kristen Engberg, who came over from Greenpeace International in 1997.